Many patients with hypertension have abnormal diastolic relaxation of left ventricular myocardium causing overt congestive heart failure despite normal systolic contractile function. Myocardial cellular calcium overload may contribute to abnormal diastolic function in these patients. Therefore, we studied the effects of a calcium channel antagonist, verapamil, in hypertensive patients and compared the effects of verapamil to equihypotensive doses of nitroprusside. Ventricular function was studied using radionuclide angiography to measure relative LV volume and simultaneously measuring LV pressure using micromanometer-tipped catheters. Patients were atrially pacing to obtain similar heart rates and mean arterial pressure to compare the effects of verapamil and nitroprusside. We found that verapamil had an expected negative ionotropic effect. However, cardiac index rose slightly from 3.1 to 3.3 L/min/m2 with verapamil and this improve left ventricular performance appeared to be due to improved left ventricular filling, evidenced by a downward and rightward shift in the end-diastolic pressure- volume relation. The comparison, nitroprusside decrease left ventricular filling which resulted in a decrease in cardiac index from 3.1 to 2.7 L/min/m2. Therefore, it appears that verapamil is effective in controlling blood pressure while preserving cardiac function due to an important in left ventricular filling.